,^8 DISCOVERY REPORTS 



posterior half of the somite and the brood lamellae articulate, one on either side with the anterior 

 region of this plate near the base of each limb. Small coxal lobes are also developed on the coxae of 

 the maxillipeds in the breeding female. 



In the male the penis (Text-fig. i6e) is single but distally cleft. 



The protopodite of the tiropod is long and tapering and bears at its extremity the small pointed 

 endopodite which is tipped with a single seta; the exopodite, like that of P. honnieri, is about three- 

 quarters the length and less than half the breadth of the endopodite, and also bears a single seta at its 

 distal extremity (Text-fig. i6Z)). 



The form of the first pair of pleopods of the male is shown in Text-fig. 16 d, that of the female m 

 Text-fig. 16 h. The basipodite in both sexes is large. Its inner margin bears coupling setae, the 

 structure of which may be seen in Text-fig. 16 d; there are fourteen of these setae in the male, but 

 only eight in the female. Its outer margin bears a number of minute rounded tubercles which extend 

 along the distal half of its length. 



In the female the exopodite and endopodite of the pleopods are of similar structure, rather small, 

 with their distal extremities fringed with a few plumose setae; the endopodite is shorter and narrower 

 than the exopodite. In the male the exopodite is modified, it is traversed by a diagonal groove which 

 extends to the distal extremity which curves outwards and is tipped with a stout spine. The outer 

 margin of the distal third of the exopodite is markedly concave and densely fringed with distally 

 directed setae; the endopodite is considerably narrower and about two-thirds the length of the exo- 

 podite, its rounded end is fringed with rather short plumose setae. 



The appendix masculina of the second pleopod of the male (Text-fig. ibc) is extremely long, it 

 gradually tapers until it has reached three-quarters the length of the endopodite, then suddenly 

 narrows to a whip-like filament ; this filament extends back about as far as the tips of the plumose 

 setae which fringe the distal end of the endopodite. 



The above description and figures were made before I received Stephensen's paper (1947) in which 

 he described a new species Microarcturus scutatus. This species appears to be identical with my 

 specimens, but I cannot agree that it belongs to the genus Microarcturus, since the four anterior pairs 

 of pereiopods do not markedly differ from the hinder three pairs, that is to say they are not of the 



astacillid type. 



Stephensen's description is based on a single male specimen, 20 mm. in length, and differs only m 

 minor points from that given above. Stephensen (p. 17) states that there are only six or seven coupling 

 setae on the first pair of pleopods (whereas I counted fourteen) in the male and eight in the female. 



Large flat tubercles, which form the 'armour ' of this species, have also been described for Antarc- 

 turus drygalskii by Vanhoffen (1914, pp. 521-3 and fig. 53); their arrangement is different, however, 

 and they are absent from the post-thoracic region. Monod (1926, pp. 27-9, figs. 23-25) described 

 a rather similar type of tubercle for A. belgica, but again these do not appear on the post-thoracic 

 segments, nor are they present on the last three thoracic segments. The tubercles are not as large or 

 distally so expanded as those of Pseudidothea scutatus, and their upper surfaces appear to be toothed 



at the margin. 



Distribution. The specimens in the Discovery collections came from a single station, off Cape 

 Bowles, Clarence Island (61° 25' 30" S., 53° 46' 00" W.). Stephensen's specimen was also collected 

 from the South Shetland area off Elephant Island, both of these stations lying within the Antarctic 

 Convergence. The only other species of the genus, P. bonnieri, is found farther north, outside the 

 Convergence, in localities around the Falkland Islands; thus, the two species, both shallow-water 

 forms, are separated from each other by the deep waters of the Drake Strait, as well as by the changed 

 hydrographical conditions on either side of the Antarctic Convergence. 



